The present invention relates to a reactor core in a nuclear reactor and to an initial core in a nuclear reactor. More specifically, the invention relates to a reactor core in a nuclear reactor and to an initial core in a nuclear reactor, that are adapted to a boiling water reactor.
The boiling water reactor has a core which is constituted by many fuel assemblies arranged in the form of a lattice. The cooling water that works both as a coolant and a neutron moderator flows into the core from the lower end of the core, heated and is vaporized as it ascends through the fuel assemblies. The cooling water inclusive of the vapor flows out from the upper end of the core. In such a core, the void fraction becomes high toward the upper region of the core, and the power distribution in the axial direction swells downwards. There has been proposed in U.S. Pat. No. 4,229,258 a fuel assembly which is capable of flattening the power distribution in the axial direction. It needs not be pointed out that the core constituted by the fuel assembly exhibits power distribution that is markedly flattened in the axial direction.
Another condition which is important in operating the nuclear reactor includes cold shutdown margin in addition to flattening the power distribution in the axial direction. Technology for improving the cold shutdown margin has been proposed in Japanese Patent Laid-Open No. 179,391/1983 (U.S. patent application Ser. No. 360,390 filed on March 22, 1982), according to which there is disclosed a fuel assembly wherein the enrichment at the upper end thereof in the axial direction in which a peak reactivity takes place is lowered during the cold shutdown of the nuclear reactor. The cold shutdown margin corresponds to shutdown margin discussed in Japanese Patent Laid-Open No. 179,391/1983 (page 3, lower right column, lines 9-16). In the fuel assembly disclosed in Japanese Patent Laid-Open No. 179,391/1983, an average enrichment in the upper region is greater than that of the lower region like in the fuel assembly taught in U.S. Pat. No. 4,229,258).